An electric power steering apparatus (EPS) serves as an apparatus where a driving section is equipped with a motor. The electric power steering apparatus provides a steering mechanism of a vehicle with a steering assist force (assist force) by means of a rotational force of the motor, and applies a motor driving force controlled with an electric power supplied from an inverter as the steering assist force to a steering shaft or a rack shaft by means of a transmission mechanism such as gears. In order to accurately generate the assist torque of the steering assist force, such a conventional electric power steering apparatus performs feedback control of a motor current. The feedback control adjusts a voltage supplied to the motor so that a difference between a steering assist command value (a current command value) and a detected motor current value becomes small, and the adjustment of the voltage supplied to the motor is generally performed by an adjustment of a duty ratio of pulse width modulation (PWM) control. A brushless motor that is superior in durability and serviceability and has little noise, is commonly used as the motor.
A general configuration of the conventional electric power steering apparatus will be described with reference to FIG. 1. As shown in FIG. 1, a column shaft (a steering shaft, a handle shaft) 2 connected to a steering wheel 1 is connected to steered wheels 8L and 8R through reduction gears 3 in a reducing section, universal joints 4a and 4b, a rack and pinion mechanism 5, tie rods 6a and 6b, further via hub units 7a and 7b. In addition, the column shaft 2 is provided with a torque sensor 10 for detecting a steering torque Ts of the steering wheel 1 and a steering angle sensor 14 for detecting a steering angel θ, and a motor 20 for assisting the steering force of the steering wheel 1 is connected to the column shaft 2 through the reduction gears 3. Electric power is supplied to a control unit (ECU) 30 for controlling the electric power steering apparatus from a battery 13, and an ignition key signal is inputted into the control unit 30 through an ignition key 11. The control unit 30 calculates a current command value of an assist (steering assist) command based on the steering torque Ts detected by the torque sensor 10 and a vehicle speed Vs detected by a vehicle speed sensor 12, and controls a current supplied to the motor 20 for EPS based on a voltage control command value Vref obtained by performing compensation and so on with respect to the current command value.
Further, the steering angle sensor 14 is not indispensable and may not be provided, and it is possible to obtain the steering angle from a rotational position sensor such as a resolver connected to the motor 20.
A controller area network (CAN) 40 to send/receive various information and signals on the vehicle is connected to the control unit 30, and it is also possible to receive the vehicle speed Vs from the CAN 40. Further, it is also possible to connect a non-CAN 41 to the control unit 30 sending/receiving a communication, analog/digital signals, a radio wave or the like except the CAN 40 to the control unit 30.
The control unit 30 mainly comprises a CPU (including an MCU, an MPU and so on), and general functions performed by programs within the CPU are shown in FIG. 2.
Functions and operations of the control unit 30 will be described with reference to FIG. 2. As shown in FIG. 2, the steering torque Ts detected by the torque sensor 10 and the vehicle speed Vs detected by the vehicle speed sensor 12 (or from the CAN 40) are inputted into a current command value calculating section 31 calculating a current command value Iref1. The current command value calculating section 31 calculates the current command value Iref1 that is a control target value of a current supplied to the motor 20 based on the steering torque Ts and the vehicle speed Vs and by means of an assist map or the like. The current command value Iref1 is inputted into a current limiting section 33 through an adding section 32A. A current command value Irefm the maximum current of which is limited is inputted into a subtracting section 32B, and a deviation I (Irefm−Im) between the current command value Irefm and a motor current value Im being fed back is calculated. The deviation I is inputted into a PI-control section 35 for improving a characteristic of the steering operation. The voltage control command value Vref whose characteristic is improved by the PI-control section 35 is inputted into a PWM-control section 36. Furthermore, the motor 20 is PWM-driven through an inverter 37 serving as a driving section. The motor current value Im of the motor 20 is detected by a motor current detector 38 and is fed back to the subtracting section 32B. The inverter 37 uses field effect transistors (FETs) as driving elements and is comprised of a bridge circuit of FETs.
A compensation signal CM from a compensation signal generating section 34 is added to the adding section 32A, and a characteristic compensation of the steering system is performed by the addition of the compensation signal CM so as to improve a convergence, an inertia characteristic and so on. The compensation signal generating section 34 adds a self-aligning torque (SAT) 34-3 and an inertia 34-2 in an adding section 34-4, further adds the result of addition performed in the adding section 34-4 with a convergence 34-1 in an adding section 34-5, and then outputs the result of addition performed in the adding section 34-5 as the compensation signal CM.
In such an electric power steering apparatus, cases that use a motor having multi-system motor windings of a constitution to continue a motor operation even if a failure (including an abnormality) of the motor occurs, are increasing. For examples, with respect to a motor having two-system motor windings, coils of a stator are separated into two systems (a U1 phase to a W1 phase and a U2 phase to a W2 phase), and it is possible to rotate a rotor of the other system even if the failure occurs in one system and to continue an assist control.
With respect to a motor control unit and an electric power steering apparatus equipped with such a motor, a method to adjust the current command value when the failure occurs in the motor winding or the inverter has been proposed. For an example, in Japanese Unexamined Patent Publication No. 2013-159165 A (Patent Document 1), an electric power steering apparatus that is provided with two systems of a motor coil and applies the assist force by driving and controlling each one, adjusts the current command value so as to make a calorific value equivalent to or less than normal one to the other system when the failure occurs in one of the systems, suppresses heat generation, and continues the assist without lowering steering feeling. Further, in a publication of Japanese Patent No. 5387989 B (Patent Document 2), when a multi-system electric motor drive unit (motor control unit) judges that the inverter or the winding group has failed in any of the systems, it stops supplying an electric power to the inverter of the faulty system, adjusts the current command value to the normal system (instead, an input value (a feedback current) to an electric power control means may be used), and makes the inverter of the normal system compensate the electric power that has been supplied by the inverter of the faulty system. After the lapse of a fixed time from the judgement of the failure, the multi-system electric motor drive unit adjusts the current command value, gradually reduces the electric power compensated by the inverter of the normal system, and prevents abnormal heat generation.